For example, the birds were presented with a vertical tube, running down to a trap-door with an out-of-reach worm perched upon it, as well as a number of different-sized stones placed nearby.

The scientists discovered that the rooks would select the largest stone, which was heavy enough to push open the trap-door when dropped and release the snack.

And when given a selection of different-shaped stones, some of which could fit into the tube some of which could not, the rooks opted for a tool that would give them access to the treat.

Lead author Christopher Bird, from Cambridge University, said: "We have found that they can select the appropriate tools out of a choice of tools and they show flexibility in the types of tools they use."

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The test revealed that rooks can use two tools in succession to get the worm

The researchers also found the rooks could use two tools in succession - something that is described as metatool use.

They gave the birds a large stone, as well as two vertical tubes, one wide, with a small stone perched at the bottom on a trap-door, and another thin, this time containing a worm.

They found that the rooks would first drop the large stone into the wide tube, releasing the smaller stone, and subsequently drop this stone into the thin tube to free the tasty treat.

Until now, metatool use has only been seen in great apes and New Caledonian crows.

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The rooks were able to fashion wire into a hook to fish out the bucket of food

Perhaps most surprisingly, the team also revealed that rooks could modify and create new tools.

They found that the rooks would bend a piece of straight wire into a hook so that they could retrieve a retrieve a bucket laden with food from the bottom of the vertical well.

Until now, this novel tool-fashioning behaviour has only been reported for a single New Caledonian crow called Betty. But in this study, three of the four rooks spontaneously created the hook in their first trial.

Bird brains?

Mr Bird told BBC News: "It was a big surprise to find out that rooks could use tools.

"We've seen this kind of tool use in New Caledonian crows, but the interesting thing about the rooks is that they do not use tools in the wild."

Both rooks and New Caledonian crows belong to the corvids, a bird group that is renowned for intelligent behaviour.

Rooks are a member of the corvid family

However, until now, it was thought that sophisticated tool-use was limited to New Caledonian crows, a species found on the island of New Caledonia in the Pacific, which create tools to pluck grubs from holes.

Mr Bird said: "Tool use is probably very important for these crows because of their ecology - they may get a large proportion of the protein they need from these grubs.

"And it has been suggested that tool-use is a trait unique to that species that might have evolved because of ecological pressures."

But the finding that rooks can use tools now raises questions about how this special ability might have come about.

Mr Bird said: "Rooks don't have the same pressures [as New Caledonian crows]. In the wild they don't need tools - they have lots of access to other sources of food, like carrion, human rubbish, and seeds from agriculture, things like that."

The researchers said that this could mean that an ancient ancestor of the corvids might have evolved the capacity to use tools as well as a complex understanding of the physical properties of materials.

Dr Emery told BBC News: "Because they don't use tools in the wild, the question is why should they have evolved the ability to use tools in the lab and understand the properties of those objects as tools?

"Is this a form of general intelligence that has been co-opted for tool use?"

The researchers say the finding raises the possibility that other corvid species may also have possess an innate ability to use tools.

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